Asp.Net Web Api中类参数的属性始终为null

Question

在我的web api控制器中有一个如下所示的操作方法

 [HttpPost]
    [EnableCors(origins: "*", headers: "*", methods: "*", exposedHeaders: "X-Custom-Header")]
    public IReportOutput InsuranceHandlingFiles([FromBody]CaseCountsInputData caseCountsInputData)
    {

    }

此操作的参数类是

[Serializable]
public class CaseCountsInputData
{
    public string MainTitle { get; set; }
    public string YearTitle { get; set; }
    public string InsurerFileCountTitle { get; set; }
    public List<FileCount> FileCounts { get; set; }
    public int InsurerTotalCount { get; set; }
    public int GrandTotal { get; set; }
    public string TotalTitle { get; set; }
    public string GrandTotalTitle { get; set; }
}

[Serializable]
public class FileCount
{
    public string year { get; set; }
    public int InsurerFilesCount { get; set; }
}

我将此API方法称为测试目的，如下所示。我的动作以这种方式调用，我的json对象绑定到CaseCountsInputData类，但所有参数都为null。你能帮我确切地告诉我错误吗？

            $("#btnExport")
            .click(function() {
            var reportData = GetReportData();
            console.log(JSON.stringify(reportData));
                $.ajax({
                    type: "POST",
                    dataType: "json",
                    data: JSON.stringify(reportData),
                    contentType: "application/json",
                    url: "http://localhost:50773/api/export/insurancehandlingfiles",
                    success: function(data) {
                          var DocumentBody = data.Data;
                    var FileName = data.FileName;
                    dataURItoBlob(DocumentBody, FileName);
                    },
                    error: function(error,as,asd) {

                        jsonValue = jQuery.parseJSON(error.responseText);
                        alert("error" + error.responseText);
                    }
                });
            });
    });

    function GetReportData() {

        var reportModel = {
            CaseCountsInputData: {
                MainTitle: "Dosya Sayısı",
                YearTitle: "Yıl",
                InsurerFileCountTitle: "Sigortacı Dosya Sayısı",
                TotalTitle: "Toplam",
                GrandTotalTitle: "Genel Toplam",
                InsurerTotalCount: 1,
                GrandTotal: 3,
                FileCounts: []
            }
        }
        var caseCounts =[];
        caseCounts.push({
            "year": 2014,
            "InsurerFilesCount": 1
        });
          caseCounts.push({
            "year": 2015,
            "InsurerFilesCount": 4
        });
        for (var i = 0; i < caseCounts.length; i++) {
            reportModel.CaseCountsInputData.FileCounts.push(caseCounts[i]);
        }

        return reportModel;
    }